1. Field of the Invention
The present invention relates to a method of fabricating a heterojunction bipolar transistor (HBT) and, more specifically, to a method of fabricating an HBT in which a hexagonal emitter electrode is disposed in a certain crystalline direction of a substrate such that a mesa type emitter has a right-angled or negative-sloped etched surface, with the results that a distance between the mesa type emitter and a base electrode can be minimized and reproducibly controlled, and a self-aligned device with an excellent high-frequency characteristic can be embodied.
2. Discussion of Related Art
In general, a heterojunction bipolar transistor (HBT) using a compound semiconductor, such as GaAs or InP, has extensively been applied as an indispensable multifunctional device for communication because it has many advantages of a high-speed high-frequency characteristic, large current driving capability, linearity of signals, and uniform driving voltage.
When the HBT is fabricated, a distance between a mesa type emitter (or an emitter mesa) and a base electrode significantly affects the performance of the HBT. A too great distance between the mesa type emitter and the base electrode leads to an increase in parasitic base resistance, thus deteriorating the performance of the HBT.
Conventionally, an HBT using a compound semiconductor, such as GaAs, has been fabricated using a self-alignment technique in order to minimize a distance between the mesa type emitter and the base electrode.
Specifically, after an emitter electrode is formed, the mesa type emitter is etched using the emitter electrode as an etch mask. In this case, the mesa type emitter has a smaller size than that of the emitter electrode due to undercut.
Thus, the base electrode can be lifted off due to a shadow effect of the emitter electrode that protrudes outward from the mesa type emitter, and the mesa type emitter can be self-aligned with the base electrode.
Also, in the fabrication of an HBT using an InP emitter layer, an InGaAs base layer, and an InP substrate, the mesa type emitter is typically etched by a wet etching process.
This is because arsenide such as InGaAs or InAlAs, which forms an emitter capping layer or the base layer, and phosphide such as InP, which forms the emitter layer, are selectively etched by a wet etching process according to the type of etchant.
Specifically, a phosphoric acid-based etchant (e.g., a mixture of H3PO4, H2O2, and H2O) etches arsenide but hardly etches phosphide, whereas an HCl-containing etchant (a mixture of HCl and H3PO4) etches phosphide but hardly etches arsenide. This selective wet etching process makes it easy to control the etch depth as compared to a dry etching process.
However, in the case of an InP-based HBT, the profile of an etched surface (i.e., whether the etched surface is positive-sloped or negative sloped) is determined depending on a crystalline direction of a substrate.
Because the etched surface has a positive slope in a certain crystalline direction, the InP-based HBT needs more excessive undercut for emitter-base self-alignment than other typical self-aligned devices, such as a GaAs-based self-aligned device.
For this reason, in the InP-based HBT, the distance between the mesa type emitter and the base electrode is increased, and there are difficulties in maintaining the distance therebetween constant. Further, as the distance between the mesa type emitter and the base electrode varies with a position in the circumference of an emitter electrode (i.e., with a crystalline direction), the properties of the HBT become unstable.
In conclusion, conventional methods cannot sufficiently control undercut and minimize the distance between the mesa type emitter and the base electrode.